Structural studies at high pressures of fluorite, CaF_2, hedenbergite, CaFeSi_2O_6, Superhydrous phase B, Mg_<20>Si_6H_8O_<36> and Phase F, Mg_<3.35>Si_<5.51>H_<7.26>O_<18> were done using a diamond anvil high pressure cell and monochromatized synchrotron radiation at Photon Factory, National Laboratory for High Energy Physics.Pressure induced phase transition of CaF_2 from cubic fluorite structure to the alpha-PbCl_2 type structure below 8.3 GPa was confirmed.The increase of bulk modulus with increasing Fe content of Ca(Fe, Mg)Si_2O_6 was observed.The crystal structure of superhydrous phase B was determined. The effect of pressure on the crystal structure was studied up to 6.5 GPa. The bulk modulus observed was 145 GPa.The cation-disordered crystal structure of phase F was determined using a small single crystal with dimensions of 18_x24_x30 micron.